In U.S. Pat. No. 4,044,948, issued Aug. 30, 1977, there is disclosed a solar water heating package which consists of approximately eight components all hooked up and ready to be connected to solar collectors and a water heater storage tank. The disclosed combination of a heat exchanger, expansion tank, pressure relief valve, filler drain valve, primary pump, flow regulating valve, check valve and differential control provide great convenience in installation in that it is only necessary to connect the solar collectors and the water heater storage tank to this package. Everything else has been hooked up and very little plumbing or electrical work needs to be done. The heat exchanger-expansion tank is even insulated to eliminate that procedure in the field.
The present invention is an improvement over the system described above in the control system therefor. As in the previous water heating package, it uses a differential control. However, in this new embodiment, the differential control has been so designed that when the sun provides just enough heat differential between the collector fluid and the water to be heated, the collector pump will run at a slow speed while the water pump runs at full speed.
It is a well known fact that to provide heat exchange, a temperature difference must first be available. For this reason, the proportional differential control of the present invention starts up the collector pump when the temperature differential is very low, for example, about 3.degree. F. It causes the collector pump to run slowly, thereby maintaining a differential adequate for heat transfer. At this time, even though the collector pump is running slowly, it is advisable to have the water pump run at full speed, increasing fluid velocity in the secondary circuit and thereby helping to further increase the overall .DELTA.T and result in still greater heat transfer. Now, as the sun gets warmer and the difference in temperature in the solar collector and the water temperature raises, for example, to about 12.degree. F., the collector pump will then be running at full speed as well as the water pump. In this way a maximum amount of heat may be picked up and delivered to the heat exchanger. By using a dual output proportional control, efficiency is increased because while the collector circuit pump may run at low speed, the secondary circuit pump will be running at high speed for maximum heat transfer at all times that the primary pump is running. Likewise, when the collector has warmed up due to the intensified heat from the sun, the .DELTA.T in the heat exchanger will be then high enough that the speed of the collector pump may increase and thereby provide greater heat transfer. The water pump runs at full speed at all times that the collector circuit pump is on at all.
The control varies the speed of the collector pump gradually from a slow speed when the .DELTA.T is approximately 3.degree. F up to full speed when the .DELTA.T is perhaps 11.degree. or 12.degree. F. However, at all times the water circuit pump is running at full speed whenever the collector pump is running at any speed. Thus, we have maximum heat transfer through the heat exchanger. It is understood that the temperature difference of 3.degree. F. or 11.degree. or 12.degree. F. are used only as illustrations and that other .DELTA.T's might be used.